In some automatic welding apparatuses, arc sensing is carried out to manipulate a welding electrode along the welding line determined by a design, deviations from the predetermined path in the up or down directions and right or left directions are detected by variations of the welding current, and the position of the welding electrode is controlled so that deviations are corrected.
An example of the arc sensing as mentioned above is explained hereinafter.
FIG. 1A shows an example of weaving in an automatic welding apparatus, wherein the welding groove 10 is viewed from above. FIG. 1B shows a cross section of the above welding groove 10, and FIG. 1B shows a MIG (metal insert gas) welding case, or a MAG (metal active gas) welding case. In FIG. 1B, reference numeral 11 is a gas nozzle, reference numeral 12 is a tip, and reference numeral 6 is a welding wire (corresponding to the aforementioned welding electrode) reference numeral 7 shows base materials, reference numeral 8 is a flow of a shield gas, and reference numeral 8 is a flow of a shield gas, and reference numeral 9 shows an arc. Arrows pointing to the right and to the left at the center of FIG. 1B, indicate the directions of the weaving.
Assuming that the welding electrode moves from point A to point B, then, as readily understood from FIG. 1B, when the welding electrode (welding wire 6) moves from the right and the left, the distance between the tip and the base material varies.
In MIG welding or MAG welding, welding machines with a constant-voltage characteristic are usually used, wherein the wire extension varies according to the variation of the tip-base material distance, and therefore, the welding current varies, i.e., a characteristic is shown wherein the greater the tip-base material distance, the smaller the welding current, and the smaller the tip-base material distance, the greater the welding current. FIG. 2 shows the variation of the welding current when the welding electrode is moved from left to right, e.g., from the point A to the point B shown in FIG. 1A, in the welding groove having a cross section as shown in FIG. 1B. The welding current is great in the vicinity of point A and point B, whereat the distance between the base material and the tip is small.
Next, with reference to FIGS. 3A, 3B, 3C, and 4, the process of detection and correction of deviation to the right or left of the welding electrode from the predetermined path by data of the welding current distribution for a half cycle of weaving, as shown in FIG. 2, is explained hereinafter. Only the case wherein the cross section of the welding groove is symmetric in the right and left directions (i.e., the welding groove is symmetric with respect to the bisector of the groove angle), is explained here.
First, at step 71 in FIG. 4, the welding corresponding to a half cycle of weaving, for example, from point A to point B, is carried out. Assuming that the welding electrode is at point A at a time t.sub.1, and the welding electrode is at point B at a time t.sub.2, the distribution of FIG. 2 is as indicated by FIG. 3A, where t.sub.c equals (t.sub.1 +t.sub.2)/2, i.e., t.sub.c is a half time of the half cycle of the weaving, where the half cycle corresponds to the time duration from t.sub.1 to t.sub.z, and at the time t.sub.c, the welding electrode (wire) is located above the center line of the welding groove. S.sub.L is an integral of the welding current for the duration of the welding of the left half of the welding groove, and S.sub.R is an integral of the welding current for the duration of the welding of the right half of the welding groove.
After the welding of the half cycle of weaving mentioned above is completed at step 72, if all welding processes have not been completed, the minimum value I.sub.min of the welding current detected during the half cycle of weaving, is obtained as shown in FIG. 3A. The difference between the welding current shown in FIG. 3A and the minimum value I.sub.min (processed welding current data) is integrated for each of the left side and right side respectively, and thus a left side integral S.sub.L, and a right side integral S.sub.R ' are obtained in step 74.
In step 75, the S.sub.L ' and S.sub.R ' are compared, and then, a part of the abovementioned processed welding current data which contributes to making one of the S.sub.L ' and S.sub.R ' larger, corresponding to a predetermined .alpha.% of the time duration from t.sub.1 to t.sub.z, is ignored, and the remaining time duration ((t.sub.2 -t.sub.1).times.(1-.alpha./100)) is divided into two equal parts. Accordingly, the mean value t.sub.c ' is obtained, and then by integrating the abovementioned processed welding current data for each time duration before the time t.sub.c ' and after the time t.sub.c ' , the integrals S.sub.L " and S.sub.R " are obtained, respectively. The abovementioned steps to obtain the integrals S.sub.L " and S.sub.R " will be understood by FIG. 3C, wherein the case where S.sub.L '&gt;S.sub.R ' is shown.
In step 76, it is determined whether or not the same side of the integral is still larger, even when the abovementioned data corresponding to the .alpha.% of the time duration is ignored. If the large-small relationship between the left side integral S.sub.L " and the right side integral S.sub.R ", which integrals are obtained by ignoring the .alpha.% data as mentioned above, is reversed when compared with the large-small relationship between the left side integral S.sub.L " and the right side integral S.sub.R ', it is deemed that there is no considerable deviation to the right or left in the original data, and the next half cycle of welding is begun. This shows that the parameter .alpha. is a right or left deviation detecting sensitivity parameter, which determines the sensitivity of a detection of deviation of the welding electrode from the predetermined path.
In the above determination, if the large-small relationship between S.sub.L " and S.sub.R " is not reversed when compared with S.sub.L ' and S.sub.R ', it is deemed that the deviation of the welding electrode from the predetermined path to the right or left is detected, and then, in step 77, the amount of correction of the position of the welding electrode is calculated in accordance with the deviation. In the equation of step 77, .DELTA..sub.1 is a right or left, and in order to improve the sensitivity when the deviation is small, the root of .vertline.S.sub.R "/S.sub.L "-1.vertline. is obtained. Alternatively, other functions which have similar characteristics to the above can be used.
The amount of correction to the right or left .DELTA.H is thus obtained, and then, in step 78, a position correction command is output to the welding electrode. This correction is realized when driving the welding electrode [actually, driving the torch (not shown) which holds the welding electrode (wire)] for the next half cycle of weaving (step 71).
The above explanation is of the procedure used to detect a deviation to the right or left when the cross-sectional shape of the welding groove is symmetrical in the right and left directions, i.e., the welding is also carried out symmetrically with respect to the center line of the welding groove. But, when the cross-sectional shape of the welding groove, or another welding condition, is asymmetric with respect to the center line of the welding groove, the aforementioned S.sub.R and S.sub.L become asymmetric according to the aforementioned asymmetry. Assuming that, in the above case, S.sub.L &lt;S.sub.R when the welding electrode has passed the predetermined path, then, if an appropriate parameter p, and S.sub.L and SR=S.sub.R .times.p/100 are used instead of the S.sub.L and S.sub.R used when the welding condition is symmetric with respect to the center line of the welding groove, the detection of deviation to the right or left can be carried out in the same manner as when the welding condition is symmetric. This means that p is an offset parameter for carrying out arc sensing during a welding when the welding condition is asymmetric to the right and left (i.e. asymmetric with respect to the center line of the weld groove), and this parameter should be determined in advance by experiment or the like.
Next, with reference to FIG. 5, a process of detecting and correcting deviation of the welding electrode from the predetermined path in the up or down direction, is explained.
In step 81 of FIG. 5, an integral of the welding current for a half cycle of weaving is obtained and this integral then used as a standard value S.sub.O. After the half cycle is completed in step 82, integrals S.sub.i (i=1, 2, . . .) of the welding current for the following and repeated half cycles of weaving (step 84) are successively obtained, each of the integrals S.sub.i is compared with the standard value S.sub.0, and it is determined whether the deviation is beyond .beta.% (step 87). If the deviation is not more than .beta.%, then the next half cycle of welding is carried out (step 74), but, if the deviation is more than .beta.%, the amount of correction of the position of the welding electrode is calculated according to the amount of the deviation found in step 88. The parameter .beta. is an up or down deviation detecting sensitivity parameter which determines the sensitivity of the detection of deviation of the welding electrode from the predetermined path, and .DELTA..sub.2 in step 88 is an up or down correction amount parameter which determines a degree of the amount of correction in the up or down direction.
To improve the sensitivity when the deviation is small, the root of .vertline.S.sub.i /S.sub.0 -1.vertline. is obtained, similar to step 77 in FIG. 4. Alternatively, other functions which have similar characteristics to the above can be used. Thus the command for the up or down correction amount .DELTA..sub.v is output in step 88, and this correction is realized when driving the welding electrode for the next half cycle of weaving (step 84).
The above explanation is of the procedure for detecting deviation in the up or down direction.
Further, at the beginning of the welding with arc sensing as mentioned above, it should be noted that arc is not stable just after the arc is first generated, and therefore, at the beginning of welding, arc sensing is carried out in accordance with the following procedure.
1.circle. the welding electrode is held in the dwell condition from the time T.sub.O (the time of the beginning of arc generation) to the determined time T.sub.1, until the arc becomes stable:
2.circle. detection and correction of a deviation to the right or left is begun at the determined time T.sub.2 :
3.circle. detection and correction of a deviation in the up or down direction is begun at the determined time T.sub.3 : where T.sub.0 &lt;T.sub.1 &lt;T.sub.2 &lt;T.sub.3.
As mentioned before, because, the operation of detecting a deviation to the right or left consists of a relative comparison between the right side integral with respect to the center line of the welding groove and the left side integral in that half cycle of weaving, it takes less time to obtain the stability necessary for detecting the deviation to the right or left. Nevertheless, the detection of a deviation in the up or down direction cannot be started until the arc is sufficiently stable after the initial deviation of the arc has been first corrected by the operation correcting the deviation to the right or left, because the detection of deviation in the up or down direction begins with an operation for obtaining an absolute value which is equal an integral of the welding current for a half cycle of weaving, as a standard value S.sub.0.
In the above description, T.sub.1, T.sub.2, and T.sub.3 are parameters which must be set in advance, by experiment or the like. The same is true in the case of .alpha., .beta., .DELTA..sub.1, .DELTA..sub.2, and p.
In the aforementioned automatic welding apparatus in which the arc sensing is carried out, the parameters which should be set in advance before the beginning of welding must be varied in accordance with the variations of welding conditions, because these parameters are determined by the welding conditions. During the welding, if the set parameters are judged inappropriate, the parameters must be changed even during welding.